I always get a kick out of the excuses exhibitionists come up with to explain their behavior, as though no one else is aware of their need for attention. What your partner was telling you was that she was more comfortable having her breast exposed to you and anyone else watching than having it restricted to merely someone who is involved in an intimate relationship with her. Her own "comfort" takes precedence over your discomfort at having to deal with the situation in a polite way. If she were truly interested in "comfort", she'd probably be getting a massage instead of training in Aikido. A T-shirt under her jacket would easily protect her modesty if she was truly that uncomfortable wearing a bra. I suspect your sensei will eventually either suggest that himself or have a senior woman in your dojo speak with her about it.